1. Field of the Invention
The invention relates to a method and a device for detecting the rotation of a brush-operated d.c. motor comprising a number of winding branches which are electrically connected between brushes during operation, with the aid of plates, according to the rotation angle.
2. Description of the Background Art
A brush-operated d.c. motor is frequently used to electrically drive an actuator of a motor vehicle, for example an electrically operable window pane, an electrically operable sunroof, an electrically operable hatch or an electrically adjustable vehicle seat, since such a motor is sturdy and relatively economical to manufacture.
In principle, a brush-operated d.c. motor includes a rotor, also referred to as a rotor coil or armature, which rotates relative to a stator, also referred to as a stator coil. To generate the torque, a d.c. motor of this type usually includes winding branches which are assigned to the rotor coil, are connectable to a direct current source and rotate in an energizing field of the stator. The energizing field may be generated either by a permanent magnet or, in turn, by current-carrying windings on the stator. In the latter case, a distinction is made between a series-wound motor and a shunt-wound motor, depending on whether the stator windings are connected in series or in parallel, relative to the winding branches of the rotor. To maintain the direction of rotation, the polarity of the current flowing through the winding branches of the rotor must be continuously reversed during rotation. This is done using a so-called commutator, which has a plate carrier that is non-rotationally connected to the rotor and rotates relative to the brushes connected to the direct voltage source. A number of plates, which are insulated against each other and are alternately contacted by the brushes during rotor rotation, are distributed over the circumference of the plate carrier. In each case, one winding branch is usually connected between two plates.
The revolutions of the d.c. motor or of a drive shaft assigned thereto are ordinarily detected to control or regulate motion and to detect the present position of an actuator driven by the d.c. motor. In particular, a position of the actuator must be determined on the basis of the detected revolutions if an anti-trap control is to be implemented for the driven actuator. After all, the actuator position may be used to ascertain whether the actuator is located in a danger zone upon dropping below a minimum gap size. The distance covered by the actuator, and therefore its position, is ascertained on the basis of the number of revolutions completed. A rotational speed may also be ascertained from the number of revolutions per time unit.
So-called Hall sensors are commonly used today in automotive engineering to detect the rotational speed or determine position. For this purpose a two-pole or multipole ring magnet is non-rotationally disposed on a drive axis and the periodic changes in the magnetic field resulting therefrom during one rotational motion are detected, using the Hall effect with the aid of a suitable sensor, namely a Hall sensor, and conclusions about the rotational speed or a position of an actuating element are drawn therefrom. However, the provision of a ring magnet assigned to the drive motor is disadvantageously associated with unwanted additional costs.
Optical systems are also know for detecting the rotational speed. However, optical systems cannot be used in automotive engineering, in particular in the installation area of the drive motor, since they are highly susceptible to dirt.